In a power amplifier of a cellular phone, an impedance transformation network is used to reduce the impedance of the load (often 50 Ohms) to a more optimal output impedance for an amplifier so that RF signals can be properly transmitted to the load. For a cellular phone that operates at a fixed transmission signal strength, the impedance transformation network needs to provide a fixed impedance transformation. However, for a cellular phone that operates at a number of different transmission signal strengths, such as a Code Division Multiple Access (CDMA) phone, the impedance transformation network needs to provide corresponding number of different impedance transformations.
An impedance transformation network can provide different impedance transformation using a shunt varactor as a variable capacitor. The shunt varactor may be a semiconductor varactor, a ferroelectric varactor or a microelectromechanical systems (MEMS) varactor. An advantage of a MEMS varactor in an impedance transformation network is that the response of a MEMS varactor with respect to change in capacitance is relatively slow compared to RF signals. Thus, the capacitance of the MEMS varactor is not significantly affected by the RF signals being transmitted through the impedance transformation network.
A typical MEMS varactor includes at least two separated parallel plates, a stationary plate and a movable plate. The movable plate is suspended over the stationary plate by a number of flexures that function as springs. The capacitance of the MEMS varactor is dependent on the distance between the plates. Thus, the capacitance of the MEMS varactor can be changed by moving the movable plate relative to the stationary plate, which is achieved by using electrostatic force generated from applied potential difference between the plates. In a conventional configuration, each plate of a MEMS varactor includes two electrodes. One of the plate electrodes is used to apply a control voltage to establish a particular potential difference between the plates to set the capacitance of the MEMS varactor to a desired setting. The other electrode is used to connect the MEMS varactor between the signal path on which RF signals are being transmitted and ground. In a non-conventional configuration, each plate of a MEMS varactor includes only one electrode. In this configuration, the root-mean-square (RMS) voltage of an RF signal is used to establish the potential difference between plates.
However, since cellular phones typically have a low battery voltage and a low RF power to actuate a MEMS varactor, the flexures of the MEMS varactor must be very responsive to the generated electrostatic force. As a result, the flexures of the MEMS varactor must be very thin and/or narrow. A concern with the use of such flexures is that the MEMS varactor may not be structurally stable, and thus, have reliability and repeatability issues.